Various processes taking into account the use of different fuels and objectives have become established for producing synthesis gases, and these represent the prior art today. Mention may be made of various modifications of entrained-flow gasification of dust-like and liquid fuels, fixed-bed gasification of particulate solid fuels and also fluidized-bed gasification which is increasingly being chosen for the use of biomasses.
In the technology of entrained-flow gasification, dust-like or liquid fuels are converted by means of a gasification agent containing free oxygen at pressures of up to 10 MPa and temperatures of up to 1900° C. into an H2- and CO-rich crude synthesis gas. Solid fuels here are coals of various degrees of carbonization which have been milled to fuel dust, or cokes, in particular petroleum cokes, biomasses or utilizable fractions obtained from residual and waste materials. This technology has been comprehensively described in “Die Veredelung und Umwandlung von Kohle”, Schingnitz, chapter 4.4.2, GSP-Vergasung, published by the Deutschen Wissenschaftlichen Gesellschaft fur Erdöl, Erdgas und Kohle e.V., December 2008. The introduction of the fuel dust into the gasification system which is under the stated pressure occurs by means of a fuel dust-carrier gas suspension having feed densities of 100-550 kg/m3 in the region of flow transport, as shown in patent document DD 147 188. Liquid fuels are heavy oils, tars and asphalts as relatively high-boiling products of petroleum processing and also of coal upgrading, for example low-temperature carbonization or coking. The liquid fuels are converted into a fine spray and likewise gasified by means of oxygen and water vapor in an entrained-flow stream, see Ch. Higman et al. “Gasification” chapter “Oil Gasification”, Elsevier publishers, 2003.
The hot crude gasification gas leaves the gasification chamber at temperatures of up to 1900° C. and pressures of up to 10 MPa together with the fuel ash which has been liquefied to slag and also fly ash and soot and is cooled directly by spraying in water or indirectly in a waste heat boiler. To separate out entrained dusts, in particular fine dusts, the crude gas goes through a cascade of water scrubbing systems or dry filters in order to remove the dust to a concentration of 1 mg/m3 (STP), preferably 0.1 mg/m3 (STP).
In the fixed-bed gasification of particulate fuels at pressures of up to 6 MPa, the crude gas having a temperature of 400-600° C. is, after leaving the reactor, cooled to such an extent that the entrained hydrocarbons are condensed out and precipitated. More detailed descriptions may be found in D. A. Bell et al. “Coal Gasification and its Applications”, chapter Moving Bed Gasifiers, Elsevier publishers 2011, where the fluidized-bed technology is also described in more detail.
The technology of endothermic steam reforming of light hydrocarbons such as natural gas, liquefied petroleum gas and light petroleum spirit at pressures of up to 3 MPa and temperatures of up to 850° C. in externally heated catalyst-containing cracking tubes deserves particular emphasis and is comprehensively described in the standard literature.
In all gasification processes, the H2/CO ratio formed initially in the gasification is not able to meet the requirements of downstream syntheses.
The known catalyzed shift reactionCO+H2O⇄CO2+H2ΔH=−41 KJ/molmakes it possible to alter the H2/CO ratio of 1:2 present in the crude gas to the ratio of about 2:1 required in the synthesis of methanol and the Fischer-Tropsch synthesis or further to produce technical-grade hydrogen. Industrial solutions are disclosed in the patents EP 2133308 and EP 2157156, and a comprehensive description of the reaction kinetics may be found in the Int. Journal of Scientific Engineering and Technology, July 2012, No. 3, pages 106-110, Y. J. Morabiya et al., “Modeling and Simulation of Water Gas Shift Reaction”.
There are conventionally shift plants which are made up of two stages. The crude gas which has been freed of dust leaves the water scrub or dry dust precipitation at temperatures of 180-220° C. and is heated in countercurrent to the converted gas to the light-off temperature of the catalyst of 200-300° C. and fed into a first reactor. The high evolution of heat in the shift reaction of 41.2 kJ/mol leads to strong heating in the shift reactor. To limit this, steam is added to the crude gas before entry into the reactor; this would not be thermodynamically necessary and serves purely for cooling the system. The crude gas which has been partially shifted in the first reactor is directly or indirectly cooled and fed to a second reactor to effect a further reduction in the carbon monoxide concentration so as to obtain the desired H2/CO ratio. The abovementioned addition of steam upstream of the first reactor represents a considerable cost factor and additionally requires larger reactors. The precise setting of the required H2/CO ratio is effected by means of a bypass regulation which conveys part of the unshifted crude gas past one or both reactors and adds it to the shifted crude gas. High CO concentrations lead to temperature increases in the shift reactor due to the strongly exothermic nature of the reaction and these temperatures accelerate the methanation reactionsCO+3H2⇄CH4+H2ΔH2—206 kJ/molCO+4H2⇄CH4+2H2ΔH2—165 kJ/molto an increasing extent, which leads, owing to the extremely high evolution of heat, to a further, even faster temperature rise. The shift reactor threatens to become overheated. To control the process, not only does the abovementioned additional introduction of steam have to take place but the CO content of the feed gas also has to be limited to <50% by volume (based on dry gas), which is difficult to achieve in industry. Various proposals have been made for controlling the problem. The company Lianxin Chemical Co. proposes reducing the steam/CO ratio before entry into the first shift reactor in the patents CN 101 050 391 A and CN 101 412 932 A. This proposal also requires a low entry temperature and also a potassium-promoted catalyst. In practical use, this means firstly cooling of the crude gas in order to condense out water vapor and subsequently reheating in order to introduce the crude gas in an unsaturated state into the shift reactor. The further condensation of water vapor in the catalyst bed is avoided thereby. This proposal means an energy loss and an additional outlay in terms of apparatus.
The patent document WO 2013 088 116 A1 proposes arranging cooling tubes in the catalyst bed in order to remove the heat of reaction by means of cold synthesis gas or steam and thus limit the temperature increase. Such reactors are complicated to manufacture, and uniform removal of heat from the catalyst bed is difficult. The second method is described in the patent document WO 2013 072 660 A1. As a further possibility, a significant increase in the space velocity through the catalyst bed of ≧12 500/h has been proposed in order to limit the reaction time. The subsequent second reactor corresponds to the abovementioned version with internal cooling. In the case of these proposals, too, the outlay in terms of apparatus is significantly higher.